1. Field of the Invention
The present invention relates to semiconductor laser devices, and particularly to an improvement in temperature characteristics and threshold current of AlGaInP system semiconductor laser devices.
2. Description of the Background Art
Conventionally, the AlGaInP system semiconductor laser devices have been studied and developed actively as semiconductor laser devices having oscillation wavelength around 630 nm band. Such semiconductor laser devices are used for laser pointers, line markers and the like because the 630 nm band has high spectral luminous efficiency, and they are also expected as light sources for high density optical disc system and the like because they have oscillation wavelength shorter as compared with AlGaAs system semiconductor laser devices.
The semiconductor laser devices are required to have good temperature characteristics such as maximum operating temperature and low threshold current. To improve these characteristics, it is necessary to make large a difference of band GaP energy Eg between an active layer and a p-type cladding layer constituting double hetero structure and also to make high the concentration of the p-type carrier in the p-type cladding layer, that is, to make large the band discontinuity (energy difference) .DELTA.Ec in a conduction band of the active layer and the p-type cladding layer to make the confinement of electrons good.
In the AlGaInP system semiconductor laser devices, (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystals in lattice matching with GaAs semiconductor substrates and having no strain are used as the p-type cladding layers. In the (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystals, the band gap energy Eg increases as the Al composition ratio x increases, but the p-type carrier concentration decreases as the Al composition ratio x increases as shown in FIG. 11. FIG. 11 shows the Al composition ratio x dependence of the p-type carrier concentration in the case of growing the (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystals using the Metal Organic Chemical Vapor Deposition method (MOCVD method) under the conditions of growth temperature (Tg) 650.degree. C. and the ratio Zn/III=0.6, the ratio of a supplied amount of Zn which is dopant and a supplied amount of III Group atoms, as an example.
As a result, the Al composition ratio x in the p-type cladding layer formed of (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystal is selected in the range of 0.5-0.8 to have good confinement of electrons.
However, such AlGaInP system semiconductor laser devices having p-type cladding layers with such Al composition ratio x selected in this way also involve a problem that the temperature characteristics are bad and the threshold current is high because the band discontinuity .DELTA.Ec in the conduction bands of the active layers and the p-type cladding layers is smaller as compared with the AlGaAs system semiconductor laser devices, for example.
To solve such a problem, it is disclosed in Japanese Patent Laying-Open No. 4-114486 to provide a multiquantum barrier (MQB) composed of alternately provided quantum well layers and quantum barrier layers having thickness which transmits electrons between the active layer and the p-type cladding layer. This multiquantum barrier highly reflects electrons on the basis of the interference of electrons by controlling the thicknesses, periods and the like of the quantum well layers and the quantum barrier layers. In such a multiquantum barrier, it is indispensable to form the quantum well layers and the quantum barrier layers having thickness through which electrons can be transmitted, that is, very small thickness (several atomic layers), so that the interference of electrons occurs, and also it is necessary to set the periods of the quantum well layers and the quantum barrier layers to predetermined values. Accordingly, in forming the multiquantum barrier, the thicknesses of the well layers and barrier layers must be controlled with a high accuracy, resulting in difficult manufacturing.